Design Parameters for Small Engines Based on Market Research Vikram Mittal United States Military Academy, Vikram.Mittal@Westpoint.Edu

United States Military Academy USMA Digital Commons

West Point Research Papers

Fall 9-10-2018 Design Parameters for Small Engines based on Market Research Vikram Mittal United States Military Academy, [email protected]

Follow this and additional works at: https://digitalcommons.usmalibrary.org/ usma_research_papers Part of the Automotive Engineering Commons, and the Systems Engineering Commons

Recommended Citation Mittal, Vikram, "Design Parameters for Small Engines based on Market Research" (2018). West Point Research Papers. 109. https://digitalcommons.usmalibrary.org/usma_research_papers/109

This Article is brought to you for free and open access by USMA Digital Commons. It has been accepted for inclusion in West Point Research Papers by an authorized administrator of USMA Digital Commons. For more information, please contact [email protected]. 2018-01-1717 Published 10 Sep 2018

Design Parameters for Small Engines Based on Market Research

Vikram Mittal US Military Academy

Citation: Mittal, V., “Design Parameters for Small Engines Based on Market Research,” SAE Technical Paper 2018-01-1717, 2018, doi:10.4271/2018-01-1717.

Abstract A market survey was conducted of commercially available mall internal combustion engines outperform engines with power outputs less than 10 kW. The subsequent batteries and fuel cells in regards to weight for a range analysis highlights the trade-offs between power output, Sof applications, including consumer products, marine engine weight, and specific fuel consumption. These engines vehicles, small manned ground vehicles, unmanned vehicles, are analyzed to show the benefits and disadvantages of and generators. The power ranges for these applications are different engine design parameters including fuel type, typically between 1 kW and 10 kW. There are numerous number of strokes per cycle, number of cylinders, intake technical challenges associated with engines producing pressure, and cooling strategy. A Pareto frontier analysis is power in this range resulting in low power density and high conducted to identify the top performing engines based on specific fuel consumption. As such, there is a large range of the output power and the total power system weight. engine design solutions that are commercially available in Recommended designs are presented for different ranges of this power range to overcome these technical challenges. output powers.

Introduction survey captured the dry weight, power output, and SFC of he last three decades have seen a significant growth in numerous small engines. internal combustion engine development especially in The collected data was further analyzed to identify the Tregards to increasing engine performance and lightest weight power systems over a given range of power decreasing specific fuel consumption (SFC) 1[ ]. Though the outputs. The power system weight included the weight of the bulk of these advancements are for larger automobile engines, fuel for this analysis. This analysis was used to identify engine advances have been made on smaller engines as well. These design guidance as a function of the required power. advancements are beneficial because smaller engines will continue to play a large role in different military and commercial markets [2]. While a significant portion of the energy market is moving towards batteries, several markets have Overview of Low-Power turned to small engines as a lightweight solution for providing reliable power for extended durations [3]. Technologies (<10 kW) These markets include small manned and unmanned vehicles, consumer products, marine applications, and Small engines have traditionally competed with fuel cells, generators. A common thread for these markets is that they batteries, and photovoltaics to power devices that require deal with portable systems where the weight is a key design less than 10 kW [5]. Since these power systems are typically parameter. If weight was not critical, these applications could for mobile or man-portable application, weight is a key simply use batteries and not deal with the complications asso- design parameter when selecting the appropriate ciated with an internal combustion engine. power solution. Since weight is critical for these applications, it is often An analysis of engines, batteries, fuel cells, and photo- necessary to approximate the total power system weight early voltaics was conducted to determine the lightest weight in the design process. Approximations are readily available solution over a range of powers and durations. This analysis for large engines based on automotive databases and practices; was performed for a robotic application; however, the overall however, these approximations do not readily scale down for trends can be expanded to other markets as well. The analysis smaller engines [4]. This paper sets out to develop these used standard design values and current development trends approximations and uncover general trends in the small for each technology to estimate the power system weight [6, 7, engine market through extensive market research. This market 8, 9] for a given power and energy requirement. The power

FIGURE 1 Map of lightest weight power solution for power A third major design challenge is associated with the draws between 10 W to 10 kW for durations between 1 minute engine exhaust [13]. The size and cost constraints on small and 200 hours. engines often result in the exclusion of mufflers. Additionally, emission requirements from small engines are less regulated than they are for larger automotive engines. Therefore, after- treatment technology, such as catalytic converters are often excluded. The noise and emissions create limitations on the use of these engines for certain applications, especially those that are used indoors. These issues are somewhat mitigated through new advances in after-treatment and active noise cancellation [14].

Commercial Markets for Small Engines

As shown in Figure 1, small engines are the lightest weight power solution for applications that require portable power greater than 1 kW for durations greater than a couple of hours. system weight included the weight of the power source, acces- A large number of applications fall into this range of power sories (e.g. alternator), and any necessary fuel. These values and mission durations. were compared between the different technologies to deter- The markets for small engines are limited to those that mine the lightest weight option. This analysis focused on a have mobile applications. Although numerous non-mobile power range from 10 W to 10 kW and a duration from applications, such as air conditioners, fall within these power 10 minutes to 200 hours. The results are plotted in Figure 1. and duration ranges, those devices can simply be plugged into Figure 1 shows that a battery solution is optimal for the grid, reducing the need for a self-contained power source. shorter durations and powers below 1 kW. These applications The market survey found five main categories of portable include mobile phones, remote controls, flashlights, and some applications for small engines. power tools. However, as duration increases past an hour, and The first market is unmanned vehicles, which include as power demand is greater than 1 kW, an engine solution is remote control aircraft and ground vehicles. Though these optimal. Additionally, for similar mission durations, but lower markets have traditionally used batteries, increased range and powers, a hybrid system is optimal, where an engine recharges power requirements have necessitated the use of small engines a battery bank. Fuel cells and photovoltaics are the lightest [3]. These engines have small alternators to provide electricity weight solution for power draws less than 200 W and dura- for electronics; however, the bulk of the power goes into tions greater than 10 hours. turning wheels or producing thrust. A subset of unmanned Despite being the lightest weight solution over a large vehicles is sophisticated military unmanned aerial systems range of powers and durations, small engine usage has been (UASs), which are seeing increasing usage over the last decade. limited by several major design challenges. The largest chal- Many of the smallest, lightest weight engines have been devel- lenge is associated with their inherently low efficiencies4 [ ]. oped for UASs. Efficiency losses through heat transfer and friction scale with Small manned ground vehicles are the second market for the surface area; meanwhile, the power output scales with the small engines. Though most automobiles require in excess to volume. Therefore, as the volume decreases, the proportional 50 kW, smaller, single-passenger vehicles can require less than losses due to friction and heat transfer increase. These losses 10 kW. This market includes scooters, riding lawn-mowers, result in a lower efficiency, which equates to a higher SFC. and go-carts. Additionally, some hybrid vehicles use low Since this issue is fundamentally a geometry issue, recent power engines as “range extenders” to recharge the batteries. developments in novel engine geometries allow for decreases This market is increasing and the engine technologies are in SFC. Many of these novel engine geometries are being advancing due to the increased usage and regulation of hybrid applied to rotary engines [10]. vehicles and scooters. A second major design challenge is associated with the Marine applications, including small recreational boats, fabrication tolerances of parts. Quite simply, small engines are the third market that use small engines. Boats typically require smaller, more accurate parts. As such, design toler- require significantly less power than ground vehicles, so ances are much more critical for small engines than for larger smaller boats have engines that produce less than 10 kW. Since engines, especially in regards to sealing. Tight tolerances battery usage on boats is somewhat limited due to safety result in difficulty in fabrication and increased cost11 [ ]. reasons, marine applications have historically used small However, advances have been made in regards to manufac- engines; this trend is expected to continue. Hence, the marine turing processes for smaller electrical-mechanical devices; engine market is fairly well developed. these advances have been translated into the development of The fourth market is consumer products, which small engines [12]. consists primarily of lawn mowers and larger power tools. © 2018 United States Military Academy. DESIGN PARAMETERS FOR SMALL ENGINES BASED ON MARKET RESEARCH 3

Though there are battery-powered variants of these devices, higher power and duration requirements often necessitate Market Survey an internal combustion engine. As batteries become more power and energy dense, the use of engines is decreasing. A market survey was conducted on small engines with power However, high-power commercial tools, which are used outputs between 1 kW and 10 kW. This range was selected for constantly through a work day, will still require mobile application where weight and power output are two small engines. key design parameters. The goal of the market survey was to Portable power generators are the fifth major market that develop a better understanding of the relationship between use small engines. Generators are used for providing power weight and power and to build correlations. in remote locations or as a backup power source. Critical The market research involved contacting and getting household devices require more than 1 kW and less than specification sheets from over 100 different engine companies. 10 kW, so small engines are commonly used as the power These companies were from eighteen different countries and source in generator sets. Batteries and fuel cells are starting were for the five markets discussed in the earlier section. Over to enter into these markets; however, small engines dominate 500 engines were identified; however, since multiple compa- this market. nies would sell the same engine under different names, this number was reduced to 284 unique engines. These engines were identified and classified. Note that due to the proprietary nature of certain engines, the names of the companies and Small Engine Designs engines are not included in this paper. The survey focused on the weight and power output of the The diversity of the engine markets result in a large range of engine. The power output was defined as the maximum steady- engine design requirements. For example, the generator state power that the engine could produce. The weight included market requires very efficient power conversion. Meanwhile, all necessary accessories for the engine to operate and produce the unmanned vehicle market is often willing to sacrifice the required power. Since fuel weight can quickly exceed the efficiency to make the engine lighter weight. And the dry weight of the engine for multi-hour missions, SFC was consumer product market is willing to accept heavier, less another key parameter included in the survey. SFC was typically efficient engines to minimize cost. Since there is a wide range not included on the specification sheets, so it was estimated of system requirements, there is a wide range of engine based on the engine technology; these estimations are shown designs. Broadly, these engine designs can be categorized as in a later section. Additionally, the market survey also collected being one of the following categories: 2-stroke, 4-stroke, date on the engine type (e.g. 4-stroke, 2-stroke, Wankel), and rotary. number of cylinders, fuel type, cost, and cooling strategy. A 2-stroke engine follows a simple engine cycle that couples the air intake and exhaust with the compression Results: Engine Types and expansion strokes. Combining intake with compression results in inefficiencies in both processes. Additionally, the As shown in Table 1, the selection of small engines were split combination of these strokes results in a significant decrease fairly evenly between 2-stroke and 4-stroke engines. Only a in efficiency due to scavenging losses during the gas few rotary engines were identified. The cost of the rotary exchange process. These inefficiencies compound to make engine makes it prohibitively expensive for many consumer 2-stroke engines fairly inefficient. The inefficiencies are markets. However, several rotary engines were being devel- somewhat offset by the engine producing power on every oped for military applications that are willing to accept a rotation. Additionally, 2-stroke engines are simple and higher cost to achieve a higher power density and lower SFC. require few parts. As such, they tend to be lightweight and As expected, the 2-stroke engine dominated markets low cost [15]. where weight was critical, especially unmanned vehicles. Though there are numerous exceptions, 4-stroke engines Approximately 77 percent of engines identified for unmanned are typically more efficient, more complex, heavier, and more vehicles were 2-stroke engines. The remaining 23 percent were expensive than 2-stroke engines. A 4-stroke engine separates rotary engines and 4-stroke engines, the majority of which the intake and exhaust processes from the compression and were used for larger military UASs. expansion strokes. As such, the engine avoids the issues of The 4-stroke engines dominated markets where fuel consump- a 2-stroke engine and can be tuned in such a way to make tion is critical and where emissions are regulated. the engine much more efficient. However, the increased -effi ciencies are slightly reduced by the engine only producing power on one of every two rotations, increasing losses to TABLE 1 The distribution of engine type by market friction [16]. Market 2-Stroke 4-Stroke Rotary The rotary engine is a variant of the 4-stroke engine that Consumer Products 14% 86% 0% uses an eccentric rotary design as opposed to a traditional Generators 0% 100% 0% reciprocating piston design. These engines are able to attain Marine 11% 89% 0% a low SFC while remaining compact and lightweight. However, Unmanned Vehicle 77% 19% 4% small rotary engines typically have sealing issues which require close tolerances in the design. As such, they are expen- Manned Ground Vehicle 8% 92% 0% sive and their use is limited [17]. All Markets 41% 56% 2%

In particular, all of the engines used in generators were The cost for the engines ranged on the order of tens of 4-stroke. Additionally, the majority of consumer products, thousands of dollars. marine, and manned ground vehicle engines were 4-stroke Second, engine costs for consumer products were typi- as well. These applications are also less weight constrained cally the lowest cost, since these engines are mass produced than unmanned vehicles. and go into cost-sensitive devices. In order to keep the cost down, these engines were typically 1-cylinder. Third, engines for marine and manned ground vehicle Results: Number of Cylinders applications were more expensive than other markets, aside The engines in the market survey can be broken down into from military UASs. Since these engines are used in manned being 1-cylinder or multi-cylinder. The simplicity of a vehicles, they are subject to additional design constraints in 1-cylinder engine makes it the favored engine for many appli- regards to emissions and safety. These additional constraints cations, with over 75 percent of the identified engines being result in additional cost. 1-cylinder. Additionally, for a given displaced volume (which Fourth, the cost of engines did not scale with the power scales with power output), a 1-cylinder has less surface area output. Often lighter-weight, lower-power engines cost more than a 2-cylinder engine, hence reducing the heat transfer and than a higher-power variant. However, the cost for 4-stroke friction losses. engines were typically more than that of 2-stroke engines; the However, multi-cylinder engines have advantages in cost for a multi-cylinder engine was more than that of a single- regards to balancing. The cylinders can be aligned and timed cylinder engine; and liquid-cooled engines were more in such a way to reduce the overall vibration of the engine. expensive than air-cooled engines. Additionally, 4-stroke engines work more reliably and produce more continuous power when they use a multi-cylinders, since each cylinder only produces power once every two rotations. Results: Other Design Figure 2 shows a histogram of power distributions for Parameters 1-cylinder and 2+ cylinder configurations. As expected, 1-cylinder engines are more commonly used for lower outputs The market survey identified a number of other engine design on the range of 1-5 kW. The higher power outputs, 6-10 kW, parameters including fuel type, cooling techniques, and use multi-cylinder engines. The market survey found that the boosting strategy. The engines in the survey all used similar bulk of the multi-cylinder engines consisted of cylinders that designs in regards to these parameters with the occasional the manufacture used in their 1-cylinder variant. outlier. The majority of the engines ran on 87-octane gasoline, used an air-cooling strategy, and were naturally aspirated. The majority of the engines ran on standard 87-octane Results: Cost unleaded gasoline. This fuel is readily available and higher octane fuels are not needed because small engines are not Cost data was not available for many of the engines identified knock limited. Most of the 2-stroke engines blend the gasoline in the market survey. However, from those that included cost with 2-5 percent lubrication oil since the fuel is injected data, several general trends were identified. through the cylinder walls. Due to the heavy weight associated First, the cost for engines in unmanned vehicles had a with diesel accessories, diesel engines were only used for bimodal distribution. The lower cost engines are intended for heavier engines, namely larger generators. Additionally, several hobbyist and were on the order of a few hundred dollars. The engines that are designed for military UAS vehicles run on higher cost engines were intended for military applications. JP-8, a variant of kerosene with properties similar to diesel fuel. These typically had higher power densities and lower SFC. Cooling strategies are critical for small engines because their low efficiencies manifest themselves as large thermal losses [18]. The majority of small engines are air-cooled through forced convection across fins. In particular, engines FIGURE 2 Histogram of power distributions for single- on manned and unmanned vehicles align cooling fins with cylinder and multi-cylinder configurations based on the direction of vehicle motion to maximize convection. market survey. Stationary devices, such as generators, use fans to force air over the engine body to remove heat. Marine engines take advantage of being submerged in water and use that water as the heat transfer medium. Less than 3 percent of the engines were liquid cooled, which were only used for unmanned vehicle engines that required long durations. The use of liquid cooling increases the reliability of the engine, though it does greatly increase the weight. Almost all of the engines are naturally aspirated with less than 2 percent of the engines being boosted through turbocharging or supercharging. Small engines are concerned with maintaining a low weight, and a turbocharger or a super- charger would weigh more than the engine itself. Additionally, small compressors have similar parasitic losses to small

© 2018 United States Military Academy. DESIGN PARAMETERS FOR SMALL ENGINES BASED ON MARKET RESEARCH 5 engines, hence compounding the inefficiencies. Boosted TABLE 2 Linear approximations for the engine dry weight as engines were used only on those that ran on heavy fuels, which a function of power output for different markets required boosting to vaporize the fuel [4]. Specific Power Market (kW/kg) Offset (kg) R2 Value Unmanned Vehicle 1.46 0.2 0.31 Manned Ground 0.36 6.6 0.67 Engine Weight Versus Vehicle Power Output Marines 0.24 5.8 0.14 Generators 0.26 8.6 0.12 An analysis was conducted to capture the weight of the engine Consumer Products 0.17 1.6 0.14 as a function of power output. Figure 3 shows the distribution of data points for weight as a function of the power output. and consumer products—all have very low R2 values since Though significant scatter, the general trends shows that the there is not vast differences between the different engine weight of the engine increases with the power output. models. However, the less established industries, unmanned Each of the five different markets were fitted separately vehicles and manned vehicles, have significantly more scatter to develop linear approximations for the engine weight as a due to the broad range of different engine designs. function of power. The coefficients to the linear approxima- The unmanned vehicles have the highest specific power tions are given in Table 2, where the weight of the engine can and lowest offset due to the fact that the majority of these be calculated by: engines are used on small aircraft which are weight constrained. power A heavier engine would necessitate additional power for flight, WeightO=+ffset (1) specific power hence creating an undesirable feedback loop. Therefore, the engines are made with the intent of reducing weight. For The weights include the balance of plant, which is substan- example, since they are used on moving flight, the engines are tially different for the each application. For example, the engines cooled through forced air convection, reducing the amount of for manned ground vehicles include significantly more after- cooling accessories required. Additionally, the engines have treatment than their unmanned counterparts. Many of the very small alternators, relying on the mechanical work extracted engines additionally require alternators to produce electricity from the engine to turn propellers and drive transmissions. from mechanical energy. The weight of the alternator can readily A high specific power was found to correspond with outweigh other components, especially for the generator market, higher costs. The highest cost engines are those associated where all the mechanical energy is converted into electricity. with military unmanned vehicles, which must achieve very The R2 values listed in Table 2 show the “goodness” of demanding power densities. Meanwhile, the lowest cost each linear fit, where a lower R2 value means that the collected engines are those associated with consumer products, which data better matches the linear approximation. Note that tend to have a very low power density. though Figure 3 appears to have a fairly tight fit between the Engines for small manned vehicles, generators, and unmanned vehicle data and the approximation, there is still marine engines all have similar specific powers and offsets. significant scatter that is not seen due to the scaling of the Though the accessories required for these three markets are y-axis. The more developed industries—marine, generators, fairly different, they are comparable in weight. For example, the weight of the alternator in the generator is comparable to the after-treatment required for a manned ground vehicle. FIGURE 3 The distribution of data points for weight as a function of power requirement for small engines in different markets. Specific Fuel Consumption Versus Power Output

Only 20 percent of engine specification sheets include the SFC or efficiency. The SFC that was included on the specification sheets is normally the fuel expenditure rate to produce power at maximum brake torque timing. The available data was used to build an approximation for the SFC of engines that did not include a value on their specification sheet. Though the SFC is based on a number of engine parameters, an analysis of variance found that the primary variables of significance was the power output divided by the number of cylinders. These results make sense because as discussed previ- ously, 4-stroke engines tend to be more efficient than 2-stroke engines, and hence have a reduced SFC. Cylinder size also plays a significant role; as the cylinder gets smaller, the surface area

FIGURE 4 Specific Fuel Consumption with approximations FIGURE 5 Pareto Frontier analysis to identify the lightest as a function of power per cylinder weight engine solution for a given power output. The weight of the power system includes the engine, accessories, and 10 kWHr of fuel.

to volume ratio increases. Friction and heat transfer scale with surface area, while power output scales with volume. Therefore, as the cylinder gets smaller, the efficiency decreases and the SFC increases. The size of the engine cylinder(s) scales with the power output divided by the number of cylinders. Figure 4 shows an approximation for SFC as a function Optimal engines identified by the Pareto Frontier of power per cylinder for 2-stroke engines and 4-stroke TABLE 3 analysis displayed in Figure 4. engines based on the data collected in the market survey. As expected, 4-stroke engines have a lower SFC than 2-stroke Dry SFC Number Number engines. Additionally, as the cylinder size increases, the SFC Power Weight (kg / of of decreases. The linear fit was used to create a correlation that # (kW) (kg) kWHr) Fuel Cylinder Strokes Market could be used to approximate the SFC for engines that did not A 1.0 1.4 0.35 Gas 1 4 UAS RC include that value on the spec sheet. B 1.5 1.5 0.35 Gas 1 4 Aircraft RC C 3.1 1.5 0.37 Gas 1 4 Aircraft Heavy D 3.7 2.4 0.31 Rotary Rotary UAS Lightweight Engine Fuel RC Design Practices E 4.5 2.5 0.33 Gas 1 4 Aircraft Gas + A Pareto frontier analysis identifies optimal solutions over F 6.0 3.8 0.41 2 2 UAS Oil a multi-variable design space. This analysis technique was Gas + RC applied to the market survey data to identify engine technolo- G 8.9 3.1 0.45 2 2 Oil Aircraft gies with the lowest system weights for given power ranges. Note that the system weight includes the engine itself, the balance of plant, and 10 kWHr of fuel. The fuel weight allows the 10 kWHr fuel weight becomes less of an issue as the engines the analysis to capture the SFC of each engine. By plotting get larger. For the larger engine, the goal is to maximize the the values, the Pareto frontier can be readily determined as engine dry weight, even at small sacrifices to efficiency. As such, shown by the dotted red line in Figure 5. The associated the smaller engines tend to be 4-stroke engines, which have a engines that were selected are shown in Table 3. lower SFC, while the largest engine is a 2-stroke with a higher SFC. All of the selected engines are used in unmanned vehicles, The outlier for the engines is Engine D which is a heavy- with several of them specifically being military UAS engines. fuel rotary engine. The engine was designed for military UAS As discussed earlier, military applications are often willing to applications, hence the need for it to run on heavy fuels. The accept higher costs to achieve higher power densities and use of the Wankel cycle allows for the engine to have a low lower fuel consumptions. Lower SFC can result in longer SFC, even at low power outputs. The low SFC makes this mission durations for a given weight, a desirable outcome for engine the optimal solution for a narrow range of powers, many military use cases. approximately 3.1 kW to 3.7 kW. However, since the power The analysis found that for smaller engines, the weight of density of the engine is significantly less than traditional the fuel plays a substantial role; hence it is important for the engines, 2-stroke engines work better at lower powers, and smaller engines to have a low SFC. For example, the fuel accounts 4-stroke engines work better at higher powers. There is poten- for 75 percent of the total weight of Engines A and B. Therefore, tial for rotary engines to dominate the lower power ranges a small decrease in SFC will significantly reduce weight. The due to their low SFC. However, many low power applications larger engines naturally have a higher dry weight. Accordingly, will not use rotary engines due to their cost. © 2018 United States Military Academy. DESIGN PARAMETERS FOR SMALL ENGINES BASED ON MARKET RESEARCH 7

6. Breeze, P., Power Generation Technologies (Waltham, MA: Conclusions Elsevier, 2014), 124-136. 7. Jha, A., Next-Generation Batteries and Fuel Cells for Internal combustion engines were compared to batteries and Commercial, Military, and Space Applications (Boca Raton, fuel cells to determine the lightest weight power solution over FL: CRC Press, 2012), 85-96. a range of power and mission times. The engine solution was found to be lighter than batteries or fuel cells for applications 8. Heywood, J. and Sher, E., Two-Stroke Cycle Engine: It's Development, Operation and Design (Boca Raton, FL: CRC requiring more than 1 kW for mission durations greater than Press, 1999), 16-28. 1 hour. As such, a number of small engines are available that range from 1 kW to 10 kW for a range of applications including 9. Heywood, J., Internal Combustion Engine Fundamentals consumer products, generators, small manned ground vehicles, (Singapore: McGraw-Hill, 1988), 702. marine vehicles, and unmanned vehicles. There are numerous 10. Hosseinalipour, S. and Delpisheh, M., “Thermal Modeling of technical challenges associated with developing engines in Novel Rotary Engines,” J. Braz. Soc. Mech. Sci. Eng. 40:4, 2018. these low power ranges; in particular, small engines typically 11. Mott, R., Machine Elements in Mechanical Design Fifth have low specific fuel consumptions and low power densities. Edition (Boston: Pearson, 2014), 489. A market analysis identified methods that companies use 12. Zhao, Y., Qiu, A., and Qi, J., “Some Important Problems and to overcome these technical challenges. The market data found Progress in Micro / Nano-Scale Thermal Science and trends between power output and engine weight for each Engineering,” Therm. Sci. Eng. 1(1), 2018. consumer application. The analysis presented approximations 13. Office of the Federal Register, Code of Federal Regulations, for the engine weight based on the desired power output for Protection of Environment (Washington, D.C.: United States each market. These approximations can be used by engineers Government Printing Office, 2011). early in the design process to estimate the power system weight. 14. Tripathi, G., Dhar, A., and Sadiki, A., “Recent Advancements A Pareto Frontier analysis was conducted to identify the in After-Treatment Technology for Internal Combustion lightest weight powers system, defined as the engine, accessories, Engines-An Overview,” Adv. ICE Res. 159-179, 2018. and 10 kWHr of fuel. The selected engines were analyzed to identify optimal design techniques. The analysis found that for 15. Taylor, C., The Internal Combustion Engine in Theory and power outputs below 5 kW, specific power can be sacrificed for Practice (Cambridge: MIT Press, 1968), 362. a lower specific fuel consumption; hence, an optimized 4-stroke 16. Taylor, Internal Combustion Engine, 363. engine is optimal due to its lower specific fuel consumption. 17. Cole, D., “The Wankel Engine,” Scientific American 227:14- However, for power outputs greater than 5 kW, the dry engine 23, 1972. weight plays a more critical role, hence a lightweight 2-stroke 18. Sonntag, R., Borgnakke, C., and Wylen, G., V., Fundamentals engine with a reasonable specific fuel consumption is optimal. of Thermodynamics (New York: John Wiley & Sons, 1998), 326.

References Contact Information 1. U.S. Environmental Protection Agency. Trends Report. The author can be reached at Light-Duty Automotive Technology, Carbon Dioxide [email protected]. Emissions, and Fuel Economy Trends: 1975 through 2013. EPA-420-R-13-011. Washington, D.C.: United States Government Printing Office, 2013. Definitions/Abbreviations 2. Wilson, K., “UAVs Graduate beyond Lawnmower Engines,” Popular Mechanics, June 8, 2010, accessed December 5, 2014, SFC - Specific fuel consumption http://www.popularmechanics.com/military/a5838/uavs- UAS - Unmanned aerial system overdue-engine-upgrade/. unmanned vehicle - Autonomous or remote-controlled vehicle 3. Hageman, M. and McLaughlin, T., “Considerations for manned ground vehicle - Vehicle used for moving a passenger Pairing the IC Engine and Electric Motor in a Hybrid Power (e.g. scooter, go-cart) System for Small UAVs,” 2018 AIAA Aerospace Sciences Meeting, AIAA SciTech Forum (AIAA 2018-2132). marine vehicle - Vehicle used to move over bodies of water 4. Ju, Y. and Cadou, C., Microscale Combustion and Power consumer products - Products used for completing household Generation (New York: Momentum Press, 2014), 425-458. tasks (e.g. snow blower, power tools, lawn mower) 5. Linden, D. and Reddy, T., Handbook of Batteries (New York: Generator - System that produces electricity typically used McGraw-Hill, 2010), 35.1-35.9. for remote or backup applications

This is the work of a Government and is not subject to copyright protection. Foreign copyrights may apply. The Government under which this paper was written assumes no liability or responsibility for the contents of this paper or the use of this paper, nor is it endorsing any manufacturers, products, or services cited herein and any trade name that may appear in the paper has been included only because it is essential to the contents of the paper.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper. ISSN 0148-7191